How to evaluate mathematical expression - Appcelerator Titanium

app.js

var win = Ti.UI.createWindow({backgroundColor : 'white'});
 
var txtFld = Ti.UI.createTextField({
    height : 40,
    width : 120,
    backgroundColor : 'gray'
});
win.add(txtFld);
 
var numLbl = Ti.UI.createLabel({
    top : 20,
    height : 40,
    width : 120,
    borderColor : 'gray',
    color:'black'
});
win.add(numLbl);
 
txtFld.addEventListener('change', function(e) {
    try{
        var evaluatedValue = eval(this.value);
    }catch(e){
        Ti.API.info('error');
    }
    numLbl.text = evaluatedValue;
});
win.open();